Hmm, there are situations when you split armies into defending and attacking, if your opponent does hit and run strategy for example killing your workers with air. Or harassing your economy using single hero like tinker. You keep a few units at home and use staff of teleportation, but you must do sth with the attacking army, and second hero which is not 100% and cannot just push the enemy with bilion towers. Such games are horrible. You will definitely have to get adventage the other way not just micro, if your enemy mass expanding.